Chemiresistors are devices which have the electrical characteristics of a resistor whose conductivity is modulated by the nature of the various chemical species with which they are in contact. Since the devices are both uncomplicated and sensitive to their environment, they have substantial potential applications both in detection, and in process control. Basically, chemiresistors comprise electrodes in contact with a semiconductor film. During use, a small voltage is supplied to the electrodes, generating a current whose magnitude depends upon the conductivity of the film, the latter in turn depending upon the nature of the ambient chemical substances with which the film is in contact.
Although the electrical conductivities of metal oxide semiconductors are detectably influenced by gases and vapors contacting their surfaces, they suffer from the fact that the heat required to operate them requires substantial power to produce, as well as from the fact that they lack sufficient sensitivity to discriminatingly detect the presence of very dilute materials.
The shortcomings of the metal oxide semiconductors has led to considerable interest in finding satisfactory organic semiconductors. While it has long been known that the conductivity of certain organic films is affected by the chemical substances with which they are in contact, the extreme sensitivity of the films to degradation has been a problem. Furthermore, the conductivity of many such films is not high, which makes it difficult to measure differences resulting from contacted substances sufficiently accurately.
Metal-substituted phthalocyanines are an exception, however, since they are capable of achieving acceptable conductivities, and in addition, such compounds do not tend to decompose readily even at elevated temperatures. Films of copper tetracumylphenoxy phthalocyanine, for example, have been formed and deposited on electrode quartz substrates to prepare gas detection chemiresistor devices.
While such phthalocyanine compounds can be used in conjunction with L-B techniques, they tend to suffer from the hydrophobic nature of their ring structure. As will be more fully explained in the following, the Langmuir-Blodgett technique consists of depositing a film-forming material on a water surface, compressing the molecules of the material together to form a compact monomolecular film, and then passing the object to be coated through the film, causing adherence of the film to the object. In the case of the phthalocyanines, the hydrophobic nature of the ring structure strongly resists deployment of the rings parallel to the surface of the water, the rings preferring to orient themselves on their edges, which results in their disposition on the surface of the water at an angle other than 0.degree.. Since the rings of the molecules are compressed together during the film-forming procedure, the rings tend to overlay each other, a configuration maintained during the subsequent coating process. The overlayed condition of the rings makes it impossible for the topmost molecules comprising the coating to fully present their gas-sensitive rings to the ambient environment, seriously impairing their detection capabilities.
In an effort to overcome this problem, hydrophilic side groups have sometimes been substituted along the periphery of the rings. While such expedients allow disposition of the rings parallel to the surface of the water, the side groups tend to interfere with the necessary L-B compression step, leaving voids in the film. Such voids both weaken the film and interfere with the electrical function of the chemiresistor sensor chips made therefrom.
Other methods which have been suggested for preparing coating films include spin-coating, as well as sublimation processes. These also suffer from a number of drawbacks, however, including the fact that films so formed are relatively thick, and they also tend to lack uniformity. In order to be successful in chemiresistor applications, it is imperative that the coating films be uniform to avoid variations in their electrical characteristics. In addition the films must be as thin as possible in order to allow the detected substances to be adsorbed rapidly and to be driven off rapidly by heating during the regeneration cycle.